RF power amplifiers are commonly used in RF circuits as the last active stage in RF transmitters. As a result, an RF power amplifier is typically the largest power consumption device in an RF system; therefore, RF power amplifier systems are designed to be as efficient as possible. One commonly used technique for improving the efficiency of an RF power amplifier is to feed the DC supply voltage of the RF power amplifier with a DC to DC converter, such that the DC supply voltage is adjusted to allow the RF power amplifier to amplify the RF signals to be amplified properly in an efficient manner.
For the DC to DC converter to output the appropriate DC supply voltage, it must be provided with an input signal representative of the desired output voltage, which is determined from the magnitude of the RF signals being amplified. By using an RF power detector, the magnitude of the RF signals can be measured.
FIG. 1 shows a typical RF power amplifier system using a DC to DC converter 10. The RF output of an RF power amplifier 12 is coupled into an RF power detector 14, which creates a DC voltage representation of the detected RF signal, which is then fed into a decision circuit 16. The decision circuit 16 then creates a control voltage for a DC to DC converter 18 using the signal from the RF power detector 14 and a stable, accurate DC reference voltage, called VREF. The DC to DC converter 18 is powered from a DC supply 20, which may be a battery.
FIG. 2 shows an RF power amplifier system 22 including a delay circuit. The RF input to an RF power amplifier stage is fed into an RF coupler 24, which extracts some of the RF signal to feed an RF envelope detector 26. The RF envelope detector 26 creates a DC representation of the RF input signal to be used by a decision circuit 28, which creates a control voltage for a DC to DC converter 30 using the signal from the RF envelope detector 26 and a stable, accurate DC reference voltage, called VREF. The DC to DC converter 30 provides the controlled DC supply voltage, called VCC SUPPLY, to an RF power amplifier 32. The DC to DC converter 30 is powered from a DC supply 34, which may be a battery. It is common for RF envelope detectors to introduce some delay in converting an RF signal into a DC representation; therefore, a delay network 36 may be needed in the RF signal path between the RF coupler 24 and the RF power amplifier 32 to preserve the linearity of the RF power amplifier 32. If a delay network 36 is needed, then the RF coupler 24 must be connected to the RF input instead of the RF output of the RF power amplifier system to compensate for the delay in the RF envelope detector 26.
A typical envelope detector circuit 38 is shown in FIG. 3. The RF input signal is fed through a diode 40 into a parallel resistor 42 and capacitor 44. The DC output is taken from the parallel resistor 42 and capacitor 44.
FIG. 4 shows an RF power amplifier system 46 without delay. RF peak detectors respond only to the peak levels of RF signals rather than the envelope of RF signals. The RF input to an RF power amplifier stage is fed into an RF coupler 48, which extracts some of the RF signal to feed an RF peak detector 50. The RF peak detector 50 creates a DC representation of the RF input signal to be used by a decision circuit 52, which creates a control voltage for a DC to DC converter 54 using the signal from the RF peak detector 50 and a stable, accurate DC reference voltage, called VREF. The DC to DC converter 54 provides the controlled DC supply voltage, called VCC SUPPLY, to an RF power amplifier 56. The DC to DC converter 54 is powered from a DC supply 58, which may be a battery. Unlike typical RF envelope detectors, RF peak detectors may not introduce delay in converting an RF signal into a DC representation; therefore, the RF coupler 48 may be connected directly to the RF power amplifier 56.
RF peak detectors can have difficulty operating with small signal levels or in systems using phase modulation where phase changes can introduce low peak levels of RF signals such that information can be lost; therefore, they may not be acceptable for use in certain applications.
One desirable characteristic in a DC to DC converter based RF power amplifier system is to provide both minimum and maximum operating limits for the DC supply voltage to an RF power amplifier. A maximum operating limit makes sure the RF output power from an RF power amplifier does not exceed required levels so that regulatory requirements, such as those imposed by the FCC, thermal limits, and power consumption limits are met. A minimum operating limit makes sure an RF power amplifier has adequate DC supply voltage to operate properly and satisfying linearity requirements of communications standards. Typically the minimum operating limit is established by a stable, accurate DC reference voltage feeding the decision circuit. When RF input signals fall below the level established by the DC reference voltage, the DC supply voltage is maintained at its minimum level. Reference voltage circuits typically require complementary transistor technology to implement, such as both n-type and p-type, which restricts the type of technologies that can be used.
An RF coupler has the characteristic of extracting some of the RF signal from a signal path, which could provide undesirable loading of RF circuits feeding the RF coupler.